1. Field of the Invention
The present invention relates to a combustion-condition diagnostic system and method for a multicylinder engine wherein the combustion condition of the engine is diagnosed on the basis of the fluctuation of revolution speeds among the individual cylinders of the engine.
2. Description of the Related Art
Any abnormality of combustion, for example, the occurrence of a misfire in an engine, forms the cause of air pollution on account of the emission of unburnt gas. Further, the unburnt gas burns in a catalyst etc. mounted for the purpose of purifying exhaust gas, so that the exhaust gas purifier portion undergoes an abnormally high temperature and has its performance lowered. As measures against these drawbacks, it is required, for example, to detect a misfire of which a motor vehicle driver is warned and to suspend the feed of fuel to the misfiring cylinder of the engine.
Prior-art techniques concerning the diagnoses of combustion conditions of engines, such as misfire, include a large number of methods; for example, a method wherein the combustion condition is detected from a fluctuation in the revolution speed of the engine, a method wherein it is detected from combustion pressure, temperature etc. in a combustion chamber, and a method wherein it is detected from the waveform etc. of a current flowing through an ignition coil. Among them, the method wherein the combustion condition is detected from the fluctuation of revolution speeds among the cylinders of the engine has the features that the rise in cost is comparatively small and that the abnormality of the combustion condition can be detected without regard to the causes (no matter which one of the fuel system, ignition system and air system of the engine may have caused the worsening of the combustion condition). A concrete example of this method is disclosed in the official gazette of Japanese Patent Application Laid-open No. 112646/1990. More specifically, the revolution speed of the engine differs between a normal combustion condition and an abnormal combustion condition. Therefore, the revolution speeds of the respective cylinders are detected at the positions of specified crank angles, and the combustion condition is diagnosed on the basis of the amount of the fluctuation of the revolution speeds among the individual cylinders.
Regarding this method, wherein the combustion condition is detected from the fluctuation of the revolution speeds, a technique intended to attain a higher precision is disclosed in, for example, the official gazette of Japanese Patent Application Publication No. 30098/1989.
This technique is intended to heighten the precision of the detection of the combustion condition in such a way that a large number of factors which bring about errors (particularly, errors in the mass of a working member, such as a piston, and in the revolution speed measurement) are nullified by evaluating the difference of kinetic energy variations in accelerating and decelerating modes of identical revolution speed (the decelerating mode needs to be one in which fuel is not burned, for example, one in which a stopping of the fuel feed is involved).
Moreover, in the method wherein the combustion condition is detected from the fluctuation of the revolution speeds, the detection precision for the combustion condition is naturally affected by the precision of means for detecting the revolution speed. Accordingly, the precision of the measurement of the revolution speed should preferably be enhanced in order to raise the detection precision for the combustion condition. Techniques for accurately measuring the crank angle in order to enhance the measurement precision for revolution speed information are disclosed in, for example, the official gazettes of Japanese Patent Applications Laid-open No. 25434/1985 and No. 92565/1991.
The former technique is such that the timing of the maximum pressure in the combustion chamber is detected by a pressure sensor on the occasion of a misfire, whereupon the detected timing signal is used for correcting the compression TDC (top dead center) signal of a crank angle sensor. On the other hand, the latter technique is such that the errors between the crank angles to-be-measured and true crank angles are fixedly acquired in relation to revolutions per minute beforehand, namely, that error components corresponding to the revolutions per minute are registered in a map or the like beforehand, whereupon the measured crank angle is corrected on the basis of the corresponding error.
Meanwhile, in the case of detecting the combustion condition from the fluctuation of the revolution speeds, when the engine revolves at high speed, a time period which is expended on the explosion and expansion strokes of the engine shortens, and the fall of the revolution speed on the occasion of the misfire is substantially in inverse proportion to the revolution speed and thus diminishes. Therefore, the difference of the revolution speeds in the normal condition and in the abnormal condition becomes slight to render the diagnosis of the combustion condition difficult. By way of example, in a case where the load of the engine is comparatively light at an engine revolution speed of 6000 [r.p.m.], the revolution speed changes only about 5.about.10 [r.p.m.] in spite of the misfire. Accordingly, when the combustion condition is to be detected even in such a high-speed and light-load state, a precision in the order of, at least, 1 [r.p.m.] is required of the information on the revolution speed.
Nevertheless, the technique stated in the official gazette of Japanese Patent Application Laid-open No. 112646/1990 does not take this point into consideration at all. Accordingly, it has the problem that the combustion condition cannot be accurately diagnosed especially at high-speed revolutions (or that revolution speed detection means of very high precision is necessitated).
In addition, since the technique of the official gazette described in Japanese Patent Application Publication No. 30098/1989 nullifies the errors on the basis of the evaluated difference between the kinetic energy variations in the accelerating and decelerating modes of the identical revolution speed, it can actually heighten the precision of the diagnosis of the combustion condition. This expedient, however, requires kinetic energy in the accelerating mode and in the decelerating mode at the identical revolution speed. The requirement poses no problem as an engine checking method which is implemented at an engine manufacturer or a service workshop. In contrast, when the expedient is to be adopted for a so-called self-diagnosis for automatically checking the engine installed in, for example, an automobile, the following problem results. Since the driving patterns of the driver of the automobile tend to deviate, an r.p.m. range in which data can be acquired on the same revolution speeds in the accelerating and decelerating modes is very narrow, and an r.p.m. region in which an accurate combustion diagnosis cannot be made comes into existence.
Besides, even when crank angle detection devices stated in the official gazettes of Japanese Patent Applications Laid-open No. 25434/1985 and No. 92565/1991 are utilized with the intention of accurately diagnosing the combustion conditions on the basis of the outputs thereof, the following problems results.
The utilization of the detection device of the former incurs the problem that the detection precision itself for the maximum pressure timing serving as the criterion of the corrections does not reach the precision which is required for the diagnosis of the combustion condition in the high-speed and light-load state as explained before, so the accurate diagnosis of the combustion condition cannot be expected in the high-speed and light-load state. On the other hand, the utilization of the detection device of the latter incurs the problem that, since the errors are fixedly set in advance, the differences of the individual engines and the change of the engine with time cannot be coped with.